Perturbation of the Degenerate, Concerted Cope Rearrangement by

Soc. , 1994, 116 (10), pp 4289–4297. DOI: 10.1021/ja00089a018 ... View: PDF | PDF w/ Links ... Journal of the American Chemical Society 2004 126 (32...
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J. Am. Chem. SOC.1994, 116, 4289-4297

4289

Perturbation of the Degenerate, Concerted Cope Rearrangement by Two Phenyl Groups in “Active” Positions of ( E ) -1,4-Diphenylhexa-1,5-diene. Acceleration by High Pressure as Criterion of Cyclic Transition States W. von E. Doering,’*’ Ludmila Birladeanu,’ Keshab Sarma,’ Joaquim Henrique Teles,’ F.-G. Klarner? and Jan-Stephan Gehrke2 Contribution from the Department of Chemistry, Harvard University, Cambridge, Massachusetts 021 38-2902, and the Institut fur Organische Chemie der Universitat-GH Essen, 0-451 17 Essen, Germany Received November 22, 1993”

Abstract: Previous examinations of radical-stabilizing substituents in the two distinct types of position in the hypothetical “aromatic” transition state of the thermal Cope rearrangement, designated ”a” or active and “n” or nodal after the allyl radical, have concentrated on their effect in the “n” positions. In order to provide a quantitatively reliable reference for the “a” position, the activation parameters of the degenerate rearrangement of (6-13C)- 1,4-diphenylhexa-l ,S-diene have been evaluated: E, = 30.8 f 0.4 kcal/mol; log A = 10.14 f 0.2. The soundly energetically-based proposition that these observations relate to a concerted mechanism is strongly supported by the observation of a 3.0-fold increase in rate of approach to equilibrium on increasing the pressure from 1 bar to 6000 bar (162 OC; benzene-d6). This rearrangement, like that of cis-l,2-divinylcyclobutaneand rac- and meso-3,4-diphenylhexa- l,S-diene, has a negative volume of activation. In contrast, trans-l,2-divinylcyclobutane,which does not rearrange by a cyclic transition state and gives cycloocta- 1,S-diene, 4-vinylcyclohexene, and butadiene as products, has a positive volume of activation. To place the possibility of reaction by the homolytic/colligative (dissociative/recombinative) mechanism on a “quantitative” base, a further sighting on the heat of formation of the cinnamyl radical is provided by activation parameters for thermal syn-anti equilibration between ( E ) - and (Z)-l,l’-bi-3-phenylcyclohex-2-enylidene:E, = 35.8 f 0.2 kcal/mol; log A = 12.7 f 0.1. After correction for conjugative interaction between phenyl and the double bond in the educts and without regard for any proposed structure for the transition state, the two phenyl groups in “a” positions appear to have lowered the enthalpy of activation by 7.7 kcal/mol relative to the paradigm, hexa- 1,s-diene, whereas the two phenyl groups in the “n” positions of 3,5-diphenylhexa-l,S-dienehave lowered the enthalpy of activation by 17.0 kcal/mol.

Many changes have been rung on the Cope rearrangement in efforts to distinguish among various mechanistic hypotheses, including the concerted, the associative diradical, and the homolytic/colligative. Despite continuous experimental, theoretical, and synthetically applied interest in the Cope rearrangement, the effect of radical-stabilizing groups at the 1, 3,4, and 6 positions of hexa- 1,s-diene has not been examined as thoroughly as might have been expected. Unlike benzene, the “aromatic”, concerted transition state of the Cope rearrangement, no matter by whom formulated, formally has two types of position as indicated in Scheme 1: two of the “n” type corresponding to the nodal, “passive” position in an allyl radical and four of the “a” type corresponding to the substituentsensitive, “active”, pair of terminal positions of an allyl radical.3 Despite the extraordinarily dramatic effects observed with oxide4 and nitrides at an “a” position, more attention has been directed toward substituents in the “n” position. Dewar3and Schmid6 were the first to explore the use of radicalstabilizing substituents at the “n” positions to engineer a qualitative e Abstract

(1) (2) (3) 4424. (4)

published in Aduance ACS Abstracts, April 15, 1994. Harvard University. Instituts fiir Organische Chemie der UniversitBt-GH Essen. Dewar, M. J. S.; Wade, L. E., Jr. J . Am. Chem. SOC.1977,99,4417-

change in the rate-determining transition state from the widely accepted, concertedchair’ (in the terms of Gajewskisand Borden9) toward that of an intramolecular, two-step, associative-dissociative passing over a cyclohexa- 1,4-diyl diradical as intermediate, the “diyl” mechanism for short.3,’ Since the pioneering work of Dewar, the phenyl group has been used so widely as the radicalstabilizing substituent that the practice is continued in this work, even though it would have been far preferable, in our opinion and, we presume, also in that of Borden? if the lead of the late Hans Schmid and his co-workers6 in employing the axially symmetrical cyano group rather than the discus-like phenyl group had been followed. The panoply of phenyl-substituted Cope rearrangements is displayed in Chart 1. Quantitative measures of the perturbations on enthalpy and entropy of activation are given in Table 1.10-12 As an example of perturbation by the phenyl group on “active” positions, 3,4-diphenylhexa-l,S-dienehas been examined in a vain attempt to realize the homolytic/colligative mechanism passing over two kinetically free and in this instance phenylstabilized allyl (cinnamyl) radi~a1s.l~Other examples include

Evans, D. A.; Golob, A. M. J. Am. Chem. SOC.1975,97,4765-4766. Evans, D. A.; Baillargeon, D. J. Tetrahedron Lett. 1978,3119-3121. Evans, D . A.; Nelson, D. A. M. J . Am. Chem. SOC.1980, 102,774-782. ( 5 ) Sprules, T. J.; Galpin, J. D.; Macdonald, D. Tetrahedron Lett. 1993, 34, 247-250.

(6) Wehrli, R.; Schmid, H.; Bellus, D.: Hansen, H.-J. Helu.Chim. Acta 1977.60. 1325-1356. (7) Dkring, W. v. E.; Toscano, V. G.; Beasley, G. H. Tetrahedron 1971, 27, 5299-5306.

(8) 6269. 6270. 6704. (9)

Gajewski, J. J.; Conrad, N. D. J. Am. Chem. SOC.1978,100,6268Gajewski, J. J.; Conrad, N. D. J. Am. Chem. SOC.1978,100,6269Gajewski, J. J.; Conrad, N. D. J . Am. Chem. SOC.1979. 101,6693-

Hrovat, D. A.; Borden, W. T.; Vance, R. L.; Randon, N. G.; Houk, K. N.; Morokuma, K. J . Am. Chem. SOC.1990, 112, 2018-2019. (10) Emrani, J. Ph.D. Dissertation, Indiana University, 1985 (Gajewski, J. J., research advisor); Diss. Abstr. B 1985,46,1922 (Order No. 85-16,636). The values given in Table 1 are our recalculation of Emrani’s experimental data. (11) Roth, W. R.; Lennartz, H.-W.; Doering, W. v. E.; Birladeanu, L.; Guyton, C. A.; Kitagawa, T. J. Am. Chem. SOC.1990, 112, 1722-1732. (12) Marvell, E. N.; Li, T. H.-c. J . Am. Chem. SOC.1978,100,883-888.

0002-7863/94/1516-4289$04.50/00 1994 American Chemical Society

4290 J. Am. Chem. SOC.,Vol. 116, No. 10, 1994

Doering et al.

Scheme 1

est

AMf + 115 kcal/mol H

H

\

\

est

H

+ 68.7

Table 1. Activation Parameters for Thermal Cope Rearrangements for Several Phenyl-Substituted Hexa-1,S-dienes Drawn in Chart 1

1,4-

rac-3,4

-

(E&)- 1,6-

dqcnwalr

compd" AH' b hexa- I ,5-dien@ 33.5 f 0.5 3-phenyl-eJ 31.5 f 0.0 3-phenylJS 28.1 i 0.4 1,3-diphenyl-* 28.2 f 0.7 1,4-diphe11yl-~ 29.9 i 0.2 1,4-diphenyl-* 32.0 f 1.9 rac-3,4-diphenylJ~~ 24.0 f 0.2 me~o-3,4-diphenyl-',~27.7 f 0.3 mes0-3,4-diphenyl-'-~ 29.0 f 0.3 2-phenyl-$ 29.3 i 1.6 2,S-diphenylJP 21.2 2,S-diphenylJ 21.3 f 0.2 2,4-diphenylJ* 24.6 f 0.8

AS'C

-13.8 -12.4 -16.8 -16.9 -15.0 -9.6 -12.4 -12.4 -10.0 -9.9 -20.8 -20.8 -16.9

T,doC 232.7 f 25.5 219.7 f 29.8 182.7 f 17.3 170.8 f 26.2 175.0 f 21.0 185.3 f 25.3 100.0 f 10.0 153.2 f 20.5 153.2 f 20.5 160.0 i 10.0 91.6 f 9 . 4 f 0.5 83.1 f 24.7 f 0.7 140.0 i 20.0

i 1.0 f 0.0 f 1.0 f 3.0 f 0.4 f 4.3 f 0.6 f 0.7 f 0.7 f 3.6

ref 7 3 3 10

p 10 13c 13d 13d 3, 12 3 11 3

All compounds are hexa-1,S-dienes and their rearrangements are degenerate except where otherwise noted. In kcal/mol. c In cal mol-' K-' (eu). d Temperature range over which kinetic measurements were made. e Gas phase. /Product is l-phenylhexa-1,5-diene. 8 In o-dichlorobenzene. In hexachlorobutadiene. In benzene-d6. In n-heptane. Product is (E,E)-1,6-diphenylhexa-l,5-diene.I In n-hexane. Product is (E,2)-1,6-diphenylhexa-I,5-diene. Product is 1,5-diphenylhexa-1,5diene. Two temperatures: no estimate of uncertainty. p This work.

*

dcgmeratr

3-phenyl-,3 1,3-diphenyl-,fO and 2,4-diphenylhexa-l,S-diene.3 From all these examples, the effect of phenyl on "a" positions might have become clear, but each falls short in one way or another: 3-phenyl for involving only a single perturbation and showing a disturbingly large difference in activation parameters between gas phase and ~ o l u t i o n ;1,3-diphenyl ~ from substantial

uncertainties in the Arrhenius parameters;*O 3,4-diphenyl from a steric factor that may translate into a large, perhaps kinetically biasing, thermodynamic driving force;l3 and 2,4-diphenyl in combining effects a t both the "a" and the "n" positions in a single compound.3 1,4-Diphenylhexa-1 J-diene seems a natural choice for focusing on the effect of phenyl in "a" positions. Placement of the two phenyl groups in identical environments has the advantage of amplifying by a factor of 2 whatever effect a phenyl group may exercise. In fact, this compound, labeled with deuterium, has already been the subject of a prior investigation,1° in which rates (13)(a) Koch, H. P. J . Chem. Soc. 1948,1111-1117. (b) Lutz, R. P.; Bernal, S.;Boggio, R. J.; Harris, R.0.;McNicholas, M. W. J . Am. Chem. Soc. 1971,93,3985-3990. (c) Lutz, R.P.; Berg, H. A. J. J. Org. Chem. 1980, 45,3915-3916. (d) Diedrich, M. K.; Hochstrate, D.; Kliirner, F.-G.; Zimny, B. Angew. Chem., In!. Ed. Engl. In press.

Rearrangement of 1,4-Diphenylhexa-l,5-diene Table 2. Specific Rate Constants and Activation Parameters for the Thermal Isomerization of (6-"(2)- 1,Cdiphenylhexa-1,Sdiene (l(6-1%))to (3J3C)- 1,4-diphenylhexa-1,5-diene (1(3-1%)) in Benzene-dh

2.54 f 0.02 6.33 f 0.08 31.3 f 0.3 65.4 f 0.5 64.9 f 1.1 65.5 f 0.7

154.0 f 0.2b 165.2 f 0.3b 186.0 f 0.2b 196.0 f O.lb 196.0 f 0.lc 196.0 f O.ld

J . Am. Chem. SOC.,Vol. 116, No. 10, 1994 4291

Scheme 2

1.00 f 0.01 1.00 f 0.03 1.00 f 0.02 1.oo f 0.02 1.00 f 0.03 1.oo f 0.00

vw:

79.1 Ea 135.4

76.6 122.6

77.5

02.4

-14.1 i 0.9 -12.8

A v+:b AV%a

Arrhenius Plot [ 1/ T vs log k ] E, = 30.75 f 0.39 kcal/mole logA = 10.14 f 0.19e Eyring Parameters

trans-2

AH*= 29.9 f 0.19 kcal/moP AS* = -15.0 f 0.8 eue

vw:

79.3 Ea 140.0

Calculated by linear regression using the usual expression for reversible first-order reaction: (kl &-I) = ( l / t ) In[(X, - Xo)/(X,- X ) ] ; K = k-l/kl. b 0.1 M solution, 0.0047 M in 18-crown-6ether. 0.05 M solution, 0.0036 M in 18-crown-6ether and 0.001 M indihydroanthracene. 0.05 M solution, 0.0036 M in 18-crown-6 ether. e 95% confidence limits. /Calculated at 175.0 OC. (I

+

of rearrangement a t various temperatures have been measured and Arrhenius parameters determined, but with insufficient accuracy to satisfy the purpose of the present study. In this work, thedegeneracy of the rearrangement is broken by I3C, a negligible perturbation on the thermochemistry and kinetics. The selected compound, (6-13C)-1,4-diphenylhexa-l,5-diene, is synthesized in the unexceptional manner described in the Experimental Section. Rates of its degenerate rearrangement in benzene-d6 are followed by IH N M R a t four temperatures in the range 154-196 OC. The primary data are offered in Table SI as supplementary material, while the derived rate constants and activation parameters aregiven inTable 2. Thecomplete absence of products expected of reaction by the homolytic/colligative mechanism, such as 1,6-diphenylhexa- 1,5-diene, supports the contention that the reaction under study is of the concerted Cope type. To gain further support for this assumption, the effect of pressure on several potential Cope rearrangements has been examined. If one expands the concept, which stems from the temperature dependence of competing single step and multistep cycloadditions, that the volumes of activation of cyclic transition states are smaller as a consequence of their greater packing fractions than those of corresponding acyclic transition states,14 then one expects of the Cope rearrangement of hexa- 1,5-diene by way of its concerted pericyclic transitionstatea negativevolume of activation and therewith a n acceleration of rate by increased pressure.13d A similarly negative volume of activation is also expected of a diradical process, provided an intermediate cyclohex1,4-diyl represents the rate-determining transition state. However, were a homolytic/colligative mechanism involved, a positive volume of activation and a deceleration of rate in response to an increase in pressure would be anticipated. As a test case for this hypothesis, cis- and trans-1,2divinylcyclobutanes (cis-2 and trans-2) have been selected as a model system (Scheme 2).15 For geometrical reasons, only cis-2 can rearrange to cycloocta- 1,5-diene by a pericyclic, concerted Cope mechanism, whereas trans-2 requires initial homolysis of the Cl-C2 bond as a prelude to rearrangement and cleavage. In fact, an increase in pressure from 1 bar to 6 kbar leads to an increase in rate of rearrangement of cis-2 to 3 by a factor of 4.5. From the pressure dependencies of the rate constants a t 69.8 O C in Table 3, an activation volume, AVI = -( 13.4 f 0.6) cm3/mol, (14) KIPrner, F.-G. Chem. 2.1983,23,53-63. Klirner, F.-G.; Krawczyk,

B.;Ruster, V.;Deiters, U. V. J. Am. Chem. SOC.Submitted for publication. (15) Hammond, G. S.; De Boer,C. D. J. Am. Chem. SOC.1964,86,899902.

A v+:C A

3

4

76.6 122.6 +4.1 f 0.4 -17.4

77.5 130.4 +4.2 i 0.7 -9.6

All values in units of cm3/mo1

a(20.0 ' C )

5 89.6 (2x448) 166.4 (2~83.2) +5.0 i 9.5 +26.4

b(69.8' C )

'(159.6 ' C )

Table 3. Pressure Dependence of the Specific Rate Constants of the Rearrangement of cis-2 to 3 at 69.8 OC in n-Heptane bar 200 400 600 800

D,

k,,x10-5 s-1 3.44 f 0.18 3.78 f 0.10 4.16 f 0.33 4.64 f 0.14

bar 1000 1500 2000 3000

D.

kl. XlO-5 s-1 5.08 f 0.13 5.65 f 0.06 6.54 f 0.12 8.53 f 0.21

Table 4. Pressure Dependence of the Specific Rate Constants of the Conversions of trans-2 to 3, 4, and 5 at 159.6 OC in n-Heptane

200 400 600 800 1000 1300 2000 3000

2.71 f 0.08 2.58 f 0.06 2.56 f 0.06 2.54 f 0.05 2.50 f 0.05 2.43 f 0.04 2.18 f 0.03 1.93 f 0.03

1.87 f 0.07 1.74 f 0.05 1.69 f 0.05 1.70 f 0.04 1.67 f 0.04 1.63 f 0.03 1.46 f 0.02 1.31 f 0.03

0.754 f 0.071 0.728 f 0.053 0.705 f 0.053 0.687 f 0.038 0.677 f 0.039 0.671 i 0.034 0.607 f 0.022 0.541 f 0.027

0.088 f 0.041 0.113 f 0.031 0.172 f 0.031 0.147 f 0.022 0.150 f 0.022 0.136 f 0.019 0.1 16 f 0.013 0.076 f 0.016

@ I n units of 10-5 s-1. bOver-all rate constant. kl(trans-2+4). kl(k3DS2-*3). e kl(tra11~2+5). can be calculated, which is in full accord with the formation of a new ring in the transition state. In contrast, the reaction of trans-2 to 3, 4, and 5 is slowed by pressure and the volume of activation becomes positive: AVI = +(4.2 f 0.6) cm3/mol, consistent with the hypothesis of a diradical mechanism. Because the product ratios, [3]:[4]:[5] ,I6 show no significant pressure dependence, the activation volumes of the individual reactions a r e essentially equal. These values a r e calculated from the pressure-dependent rate constants given in Table 4 and are shown in Scheme 2. It is concluded that neither ring closure of the diradical to 3 or 4 nor cleavage to 5 are product determining. H a d that not been the case, differing volumes of activation would have been expected. Quite probably pressure-independent rotations about theC-C bondin thediradicals6determine thedistributions among the three products. It is also of interest to compare the volumes of reaction that can be determined from the partial molar volumes (V) (Scheme 2) in the rearrangements of trans-2 to 4 ( A P = -9.6 cm3/mol) and trans-2 to 3 ( A P = -17.4 cm3/mol) in which six- and eight(16) A quantitative analysis of butadiene was not possible because of its volatility. Nonetheless the amount of butadiene is estimated, as described in detail in the experimental section,as a loss of CsHlz isomers against n-nonane as internal standard. This estimate being significantly less accurate than that of the CsH12 isomers, the activation volume for the rearrangement of tmns-2 to 5 has the greatest uncertainty.

4292 J. Am. Chem. SOC.,Vol. 116, No. 10, 1994

Doering et al.

made.22 If this value is added to the enthalpy of activation of the Coperearrangementof hexa-1,5-diene (33.5 kcal/mol, gas phase) as an apposite model corrected for stabilization by conjugative P, bar t , min rati@ kib interaction in the educt, the observed enthalpy of activation of 1 1000 16.4:23.6 5.31 l,Cdiphenylhexa-l,S-diene,29.9 kcal/mol, translates to a lower72.1:27.9 6.23 530 1093 70.3:29.7 6.88 ing of 7.7 kcal/mol in the enthalpy of formation of the transition 1000 1090 68.1:3 1.9 7.86 1530 1080 state. Each phenyl group in an "a" position has thus appeared 2020 1085 65.7:34.3 8.87 to stabilize the transition state by 3.9 kcal/mol. 2530 1088 64.8:35.2 9.33 Similar analysis of 2,5-diphenylhexa-l,5-diene,in which the 3010 1080 63.1:36.9 10.3 61.5:38.5 11.3 substituents occupy "n" positions, reveals a lowering in enthalpy 3520 1080 6000 1000 51.9:42.1 15.3 of formation of the rate-determining transition state of 17.0 kcal/ mol (33.S7 + 2(2.4)23- 21.311), corresponding to 8.5 kcal/mol * The ratio of l(6-W)to l(3-l~) as measured by IH NMR after time per phenyl group. If the conceptual scheme behind this analysis of reaction ( 1 ) . b In units of 10-6 s-1. posits both 1,4- and 2,5-diphenylhexa-1,5-dieneto proceed by a concerted mechanism, a phenyl group in an "n" position is more membered rings, respectively, are formed at the expense of a than twice as effective as one in an "a" position. If the transition four-membered ring. This observation that the volume requirestate for the Cope rearrangement resembled that for dissociation ment in the ring enlargement from four to six members is much to two allyl radicals, this result would be difficult to reconcile less than it is in that of four to eight members makes it likely that the volume of activation depends not only on the number13dbut with the finding that the bond dissociation energy of 2,5also on the size of the newly forming ring. The van der Waals divinylhexa- 1$diene (to a pair of 2-vinylallyl radicals)24 is volumes ( V W ) ~ ~and J * the intrinsic volumes of the ground and unchanged from that of unsubstituted hexa-1,5-diene;25 that is, transition states of the CsH12isomers shown in Scheme 2 do not a vinyl group in the "n" position of the free allyl radical has no differ from each other appreciably and cannot explain the observed more effect here than it does on the two double bonds in the differences in volumes of activation and reaction. educt. In analogy to the rearrangement of cis2 to 3, the degenerate The effect of the phenyl groups in the 1- and 4-positions may rearrangement of 1,4-diphenylhexa-l,5-diene reveals an acbe analyzed under an alternative hypothetical scheme, in which celeration in rate of approach to equilibrium between l(6-lT) 1,6 bond-making precedes 3,4 bond-breaking, leading to the'diyl", and l(3-IT) in response to an increase in pressure from 1 bar 1,4-cyclohexadiyl diradical as intermediate. As theunsubstituted to 6 kbar by a factor of 3.0. The volume of activation is calculated model for this comparison, a heat of formation for the cyclohexato be AV = -(9.1 f 0.5) cm3/mol from the pressure dependence 1,4-diyl diradical, by its very definition noninteractive, may be of the specific rate constants at 162 O C given in Table 5. This constructed from the heat of formation of cyclohexane (-29.5 value is similar to those found in the rearrangement of mesokcal/mol)21by addition of twice the difference in heat of formation 3,4-diphenylhexa-1,S-diene to (E,Z)- 1,6-diphenylhexa-1,S-diene of propane and isopropyl radical (+46.3 kcal/mo1).26 The (127.5 OC: AV = -(13.3 f 0.3) cm3/mol) and to (Z,Z)-1,6resulting estimate of +63.1 kcal/mol is 9.5 kcal/mol higher than diphenylhexa-1,5-diene (127.5 OC: AVI = -(8.8 f 0.2) cm3/ the experimental heat of formation of the rate-determining m01).'3~ The rate acceleration established here for the intertransition state for the concerted Cope rearrangement [+53.6 conversion of l(6-IC and ) 1(3-IT) argues strongly for the kcal/mol: the heat of formation of hexa-1,S-diene (20.1 kcal/ pericyclic mechanism. plus the enthalpy of activation (33.5 k ~ a l / m o l ) ~ ]In. other The observed enthalpy of activation of the degenerate rearwords, the energy of concert27.28 of the rearrangement of hexarangement of 1, AH* = 29.9 kcal/mol, is lower than that of 1,5-diene vis-h-vis the "diyl" as model is 9.5 kcal/mol. unsubstituted hexa-l,S-diene, AH*= 33.5 kcal/mol, by 3.6 kcal/ mol.19 Without correction for the stabilizing conjugative interac(21) Pedley, J. B.; Naylor, R. D.; Kirby, S.P. Thermochemical Data of tion in the single styrene moiety of the educt, this difference Organic Compounds, 2nd ed.; Chapman and Hall: London, 1986. (22) The difference between the known heats of formation of ethene and cannot be equated with the magnitude of the perturbation on the styrene is 22.8 kcal/mol.21 This value is taken for AAPf[RCH=CH2/ transition state. Knowledge of the conjugative interaction between RCH===CH(CsHs)] and has also been incorporated as general in storage a double bond and a phenyl group is the critical element. Perhaps schemes like Benson's or Allinger's MM. Applied to propene, M r = +4.78 kcal/mol, an estimate of the experimentally unknown heat of formation of surprisingly, the magnitude of this factor is not known with great 1-phenylpropeneof +27.6 kcal/mol is generated. Subtraction from the heat confidence because of the paucity of relevant thermochemical of formation of I-phenylpropane (+1.9 kcal/mol) affords an estimated heat data in the literature.20.21 An estimate of 4.1 kcal/mol can be of hydrogenation of 25.7 kcal/mol. This value is lower than the heats of Table 5. Pressure Dependence of Specific Rate Constants of the Equilibration 1(6-'C+ ) 1(3-'%) at 162.0 OC in Benzene-&

(17) The van der Waals volumes (VW)are the intrinsic volumes of the ground and transition states multiplied by Avogadro's number. They are calculated by a computer program MOLVOL developed by U. ArtschwagerPer1and D. Oebels,a copy of which can be obtained on rquest. This program employsCartesian coordinatesderivedfrom molecular or quantum mechanical calculations and atomic van der Waals radii (Rw) derived by crystallographic analysis (Rw(C) = 1.80 A; Rw(H) = 1.17 A). (1 8) (a) Artschwager-Ped, U. Dissertation, Ruhr-Universitit Bcchum, 1989. (b) Yoshimura, Y.; Osugi, J.; Nakahara, M. Bull. Chem. SOC.Jpn. 1983,56, 680-683. (c) Asano, T.; Le Noble, T. J. Rev. Phys. Chem. Jpn. 1973,43,82-91. (19) However, note well that the datum from hexa-1,5-dienerelates to the gas phase7J2 whereas that from 1,4-diphenylhexa-l,5-dienerefers to reaction in solution in benzene. The difference in phase might be ignored were it not for the observation by Dewar and Wade3 in the rearrangement of 3-phenyl1,5-hexadieneof a difference of 3.4 kcal/mol between the gas (AH*= 33.5 kcal/mol) and the liquid (AH* = 30.1 kcal/mol) phase (see Table 1) (4-fold difference in rate)." Were this correction applied, the apparent difference in enthalpy of activation between 1,4-diphenyl-substitutedand unsubstituted 1,5-hexadienewould vanish. (20) Included in the collection of Pedley, Naylor, and Kirby*] are heats of formation of styrene (35.35 kcal/mol) and indene (39.05 kcal/mol) and heats of combustion (liquidonly)of 2-phenylpropene,1,2-dihydronaphthalene, 1-phenylcyclohexene,and cis- and trans-l-phenyl-3,3-dimethylbut-l-ene.

hydrogenation of monoalkyl substituted olefins, including that of propene (-29.8 kcal/mol), by 4.1 kcal/mol. This is the value we currently take for the conjugative interaction of phenyl in compounds of the type rransRcH=CH(C&), but with some reservation! Note that any differences in steric factors areabsorbed into'conjugative interaction",despitethe electronic component alone being relevant to the argument. (23) A heat of hydrogenation of 2-phenylpropene, estimated to be -27.4 kcal/mol (from a heat of formation of 2-phenylpropene, itself estimated on the basis of footnote 23 in ref 11 to be 28.4 kcal/mol, and a known heat of formationof 2-phenylpropneZ1),is 2.4 kcal/mol less than that of RCH=CHz and is taken as the value for conjugative interaction in olefins of the type RC(C6Hs)=CH2. (24) Roth, W. R.; Staemmler, V.; Neumann, M.; Schmuck, C. Chem. Ber. In press. (25) Doering, W. v. E.; Roth, W. R.; Bauer, F.; Boenke, M.; Breuckmann, R.; Ruhkamp, J.; Wortmann, 0. Chem. Ber. 1991,124,1461-1470. (26) (a) Seetula, J. A.; Russell,J. J.; Gutman, D. J. Am. Chem. Soc. 1990, 112, 1347-1353. (b) Seakins, P. W.; Pilling, M. J.; Niiranen, J. T.; Krasnoperov, L. N. J. Phys. Chem. 1992,96,9847-9855. (27) "Energy of concert" translates awkwardly and ambiguously into German; Professor Roth uses the term, 'hergangzustands-Resonanzenergie".Z* (28) Doering, W. v. E.; Roth, W. R.; Bauer, F.; Breuckmann, R.; Figge, L.; Lennartz, H.-W.; Fessner, W.-D.; Prinzbach, H. Chem. Ber. 1988,121, 1-9.

J. Am. Chem. SOC.,Vol. 116, No. IO, 1994 4293

Rearrangement of I ,4- Diphenylhexa- IJ-diene Analysis of 1,4-diphenylhexa- 1S-diene proceeds analogously. Its heat of formation is estimated from that of hexa-1,S-diene by the addition of +22.8 kcal/mo122and +25.70 kcal/mol.29 To the resulting estimated heat of formation (+68.7 kcal/mol), addition of the enthalpy of activation (+29.9 kcal/mol) leads to an estimated heat of formation of the rate-determining transition state of H 8 . 6 kcal/mol. Estimation of the heat of formation of a noninteractive, 2,5-diphenylcyclohexa-1,4-diyl diradical, which shall serve as the non-concerted model in this analysis, is derived by replacing two of the methylene groups in the cyclohexa-1,4diyl diradical (+63.1 kcal/mol) by two benzyl groups (2 X 25.7 kcal/mol). From the resulting heat of formation, +114.5 kcal/ mol, an energy of concert of 15.9 kcal/mol is estimated. This value, being greater than that of the paradigm by 6.4 kcal/mol, is consistent with each phenyl group having stabilized the actual transition state, which is presumed to be that involved in the concerted mechanism, by 3.2 kcal/mol. The mean of the magnitude of the perturbation by the two analyses is 3.5 kcal/ mol. An identical analysis of 2,s-diphenylhexa- 1,S-diene leads to an essentially zero energy of concert vis-a-vis 1,4-diphenylcyclohexa-1,4-diyl as thedefined model.’’ In a futurepublication,24 Roth and co-workers will indicate as weaknesses in this analysis its neglect of changes in steric energy associated with coplanarization of the phenyl group in the delocalized diyl and its acceptance of 13.0 kcal/mol as the delocalization energy of the benzyl radical instead of the now preferable value of 10.5 kcal/ m0l.3~ Bearing witness to an inevitable reliance on currently available thermochemical data, which is the experimental complement to current inadequacies of theoretical calculations on large molecules, as the weakness in all these analyses, they will draw a conclusion opposed to the earlier one.” The homolytic/colligative mechanism has consistently seemed quite untenable for the unsubstituted paradigm, but now that the enthalpy of activation for cleavage of hexa- 1,S-diene to two allyl radicals has been demonstrated by Roth and co-workers to be 55.7 k ~ a l / m o l , ~reaction l by this mechanism is seen to lie some 22 kcal/mol above the experimental value for its Cope rearrangement.’ That it might have become tenable for 1,4diphenylhexa-1 ,S-diene has already been proposed and rejected within the limitations of a poorly defined heat of formation of the cinnamyl radical.32 In the meantime, a value for the stabilization energy of the cinnamyl radical of 17.4 kcal/mol based on work of M. Herbold has been reported by Roth et al.24and another value is derived by addition of the increment of +5.4 kcal/mol above the stabilization energy of the benzyl radical proposed by Robaugh and Stein.33 When added to the recent value for the stabilization energy of benzyl radical, 10.5 kcal/mol, advanced by Hippler and Troe,30 a stabilization energy of 15.6 kcal/mol for cinnamyl results. To these, we now add a value based on the application of the method of thermal &,trans isomerization about a double Selection of bi-3-phenylcyclohex-2-enylidenes, 7 and 8, as the substrate is motivated in large measure by its closeness in structure to the reference trienes 9 and 10 in Chart 2. Synthesis and isolation of 7 in pure form is described in the Experimental Section. Thermal isomerization is effected in solution in benzene-da at temperatures ranging from 154 to 196 OC. From the primary (29)See footnote 22 in ref 11: the difference in heat of formation of an aliphatic methylene group and an alkyl disubstituted benzyl group. (30) Hippler, H.; Troe, J. J. Phys. Chem. 1990, 94, 3803-3806. (31)Roth, W. R.;Bauer, F.; Beitat,A.;Ebbrecht, T.; Wiistefeld, M. Chem. Ber. 1991, 124, 1453-1460. (32)See particularly, p 1729 of ref 11. (33)Robaugh, D. A.; Stein, S. E. J . Am. Chem. Sac. 1986,108, 32243229. (34) Doering, W . v. E.; Beasley, G . H. Tetrahedron 1973, 2231-2243. (35) (a) Doering, W. v. E.; Kitagawa, T. J . Am. Chem. SOC.1991, 113, 4288-4297.(b) Doering, W.von E.;Sarma, K.J.Am. Chem.Soc. 1992,124, 6038-6043.

Chart 2

b

A# 35.0 kcal/mol

I

A# 38.9 kcal/mol

A P 39.6 kcal/mol

Table 6. Specific Rate and Equilibrium Constants and Activation Parameters for the Thermal Interconversion of (E)- (7) and (Z)-,l’-Bi-3-phenylcyclohex-2-enylidene 1 (8) in Benzene-ds

x10-6 s-I 2.22 f 0.03 2.11 fO.O1 6.12f 0.03 6.08 f 0.14 18.36 f 0.57 18.36 f 0.28 39.50 f 0.68 41.89 f 1.28 91.32 i 1.35 95.06i 1.63

T, OC 154.0i 0.2 154.1 i 0.2 165.1 i 0.2 165.2f 0.2 177.1 i 0.2 177.1 f 0.2 185.5 i 0.3 185.9 i 0.3 196.2f 0.2 196.3 f 0.2

&l,’’b

K 0.560 0.576 0.580 0.584 0.590 0.603 0.610 0.585 0.610 0.619

Arrhenius Plot [ 1 / T vs log k] E, = 35.84 f 0.19 kcal/mol log A = 12.66 f 0.09 Eyring Parametersc

AH* = 35.0 i 0.2kcal/mol A!?* = -3.4 f 0.4 eu Thermodynamics [ 1 / T vs log K ] AJP = 0.67i 0.09 kcal/mol ASo= 0.46f 0.21 eu Calculated by linear regression using the usual expressionfor reversible K= first-order reaction: (&I &-I) = (l/t) In[(& -Xo)/(X, k-l/kl. b Double all standarderrors for 90% confidencelimits. Calculated at 175.2 OC.

+

-a];

data, available as supplementary material in Table SII, specific rate constants and activation parameters are calculated and recorded in Table 6. The enthalpy of activation for conversion of 7 to 8 is 35.0 kcal/mol. This value may be compared with that of 38.9 kcal/mol for 935a and 39.6 kcal/mol for 1036 in Chart 2. If stabilization energy is defined strictly as illustrated for the least substituted cases in Chart 3, that is, in neglect of styrene conjugative interaction, stabilization by cinnamyl is 2.2 kcal/ mol greater than that by allyl (13.5 kcal/mol), or 15.7 kcal/mol in good agreement with the value offered by Robaugh and Stein (10.530 + 5.433). A heat of formation of the cinnamyl radical can then be derived: estimated heat of formation of trans-1phenylpropene (+4.5 22.822 = 27.6) + 48.326- 15.7 = 60.2 or 120.4 kcal/mol for two cinnamyl radicals. The estimated heat of formation of the transition state of the Cope rearrangement of 1,Cdiphenylhexa-l ,S-diene being 98.6 kcal/mol, energy of concert vis-a-vis the homolytic/colligative mechanism is estimated

+

(36)Doering, W. v. E.; Shi, Y.-q; Zhao, D.-c. J. Am. Chem. SOC.1992,

114, 10763-10766.

Doering et al.

4294 J. Am. Chem. Soc., Vol. 116, No. 10, I994 Chart 3 70

60

50

SE,

40

AH 30 kcaW mol

W*O

20

10

H

n

0

6 H

"4

-10

2SE, = d 1

-*

-

o + 2K AH$,

BE, = d

-

Q

o + 2K AH*,

I

I

K = The Kistiakowsky:The unit of enthalpy of conjugation in polyenes (3.74 kcal/mol) to be some 22 kcal/mol, a value far outside conservative estimates of uncertainty. Nonetheless, the possibility has been considered that homolysis could generate cinnamyl radicals in low steady state concentration and initiate a chain process, involvingaddition followed by internal rotation and elimination of a chain-propagating cinnamyl radical. In order to probe this possibility, 1(W) is heated at 154 "C in thiophenol as solvent. With the exception of one product of m/e 153 (addition of thiophenol to a phenylpropene leads to m/e 228), all other products separable by GC have m/e 344, corresponding to an addition of thiophenol to educt, which apparently occurs more rapidly than the Cope rearrangement. At the much higher temperature of 259 OC in the presence of 9,lO-dihydroanthracene as radical trapping agent, (E,E)- and (E,Z)-1,6-diphenylhexa-1,5-diene and phenylpropenes (mainly trans-1- but also some cis-1- and 3-phenylpropene) appear in substantial amount. Although colligationof two cinnamyl radicals can generate all the isomers of 1,6-, 1,4-, and 3,4-diphenylhexa1,Sdienes in principle, in practice neither of ihe 3,4-diphenyl isomers is seen owing to an unfavorable thermochemistry, which causes (E,E)- and (E,Z)-l,6-diphenylhexa-1,5-dieneto be the only products of the rearrangement of 3,4-diphenyl at lower temperat~res.l3~-~ Strong evidence for the incursion of the homolytic/colligative mechanism is given by the formation in substantial amount of the phenylpropenes, consistent with the product of hydrogen atom transfer by 9,lO-dihydroanthracene (see Table 7). Although these experiments could not beconducted with sufficient accuracy or reproducibility to provide reliable activation paameters, they are nonetheless in accord with the generation of cinnamyl radicals at high temperature, if by no means definitive in establishing the homolytic/colligative mechanism. A cautionary comment about the quantitative reliability of certain of the conclusions in this paper notes the surprisingly

Table 7. The Results (in 5%) of Heating (E)-1 ,4-Diphenyl-( 12-C)-hexa-1,Sdiene (1) at 259 "C in Toluene in the Presence of 9,lO-Dihydroanthracene Droducts 3-Ppo4 (2)-1 -PP ( E )- 1 -PP (2)- 1,4-DPH' (E)-1,4-DPH (E,Z)-1,6-DPH (E,E)-1,6-DPH

0.0h

>99.9

1.5 h

3.0h

6.0h

24h

3.7 1.4 15.8 7.5 67.4 1.2 3.0

6.8 2.7 27.4 4.4 41.8 3.6 13.3

10.4 4.1 40.8 2.0 21.8 4.2 16.7

11.7 6.4 54.8 10.9 3.3 12.9

a Phenylpropene. Diphenylhexa- 1 J-diene. Although concentrations of these compounds are normalized to loo%, other products, including higher oligomers, are omitted.

small number of thermochemical data in the literature relating to the phenyl group. In light of the ultimate goal of chemistry-the translation of structure of educts and products into rates of interconversion and positions of equilibria-it is hard to excuse the continuing neglect of thermochemistry. Had this work and others been carried out with cyano in place of phenyl, the situation would have been no better. Further to emphasize the gratuitous complications that have been introduced by the choice of phenyl as a ?r-electron interacting group, we note that three possibly useful definitions of conjugative interaction between phenyl and olefin and three of stabilization enthalpy in the cinnamyl radical can be considered in principle: one in which the plane of phenyl is perpendicular to the plane occupied by interacting olefin; a second in which a coplanar relation exists; and a third in which the most favorablebalance is struck between delocalization energy and steric energy at some intermediate angle. We conclude that a proper study of phenyl in this and probably other situations requires examination also of compounds like 11 and 12 in Chart 2.

Rearrangement of 1,4-Diphenylhexa-1,5-diene

J . Am. Chem.Soc., Vol. 116,No. 10, 1994 4295

chromatography column packed with silicagel in hexane. Elution with hexane gives 0.286 g (66.2%) of I3C-labeled diene: 97.4% of purity by GLC (column B); 'H NMR (C6D6) 726-7.03 (my 10 H), 6.33 (d, 1 H, J 15.8 HZ), 6.16-6.07 (m, 1 H), 6.01-5.92 (m, 1 H), 5.15 (m, 1 H), andap0logize~tothosewhosecarefulworkhasmadeth~ca~~.3~~39 4.90-4.84 (m, 1 H), 2.59-2.48 (m, 2 H). (E)-l,CDiphenyl( l-1W)hexa-1,5-diene. This compound is prepared Experimental Section by themethod of K0ch.lJa A solution of trans-cinnamyl chloride (Aldrich; 11.1 g, 0.073 mol) in 25 mL of dry ether is added over a period of 40 min General Methods. IH NMR (500 MHz) and NMR (125.8 MHz; to a stirred mixture of Mg turnings (0.9 g) and iodine (-10 mg) in 5 C.&) spectra are recorded on a Brucker AM-500 instrument. Chemical mL of ether so as to maintain gentle reflux. After being boiled under shifts are reported in ppm (6) relative to TMS. Infrared spectra (cm-1) reflux an additional hour, the reaction mixture is poured into an ice-cold, are recorded on a Perkin-Elmer Model 337 grating spectrophotometer. saturated, aqueous solution of ammonium chloride. Theseparated organic Analytical GLC is effected on a Hewlett-Packard 5890A instrument layer is washed with 5% ice-cold HC1 and water, dried (MgSO,), and with a Hewlett-Packard recorder-digital integrator H P 3393A; column concentrated in uucuo. GLC analysis of the residual pale-yellow oil on A, 7%cyanopropyl-7%phenylpolysiloxane (J & W Scientific, DB 1701), column A reveals a mixture of six compounds, retention times in min (7% 0.53 mm i.d. X 30 m operated at oven temperature 100-180 "C, He 10 yield): A, 25 (30.6); B, 29 (0.4); C, 34 (52.2); D, 35 (0.3); E, 41 (6.3); lb; column B, 100% methylpolysiloxane (J & W Scientific, DBl), 0.53 and F, 49 (10.2). The crude product (6.5 g) is distilled under reduced mm i.d. X 30 m. pressure (1 mmHg), three fractions being collected in the temperature In the high-pressure investigations, IH NMR (300 MHz) spectra are range 145-160 O C . Fraction 1 depositscrystalline meso-3,4-diphenylhexarecorded on a Brucker AMX-300 instrument. Analytical GLCis effected 1,5-diene, which, after recrystallization from EtOH, has mp 85-85.5 O C on a Hewlett-Packard 5890 instrument with a Shimadzu recorder digital (lit.I3a mp 86-87 "C) and retention time 25 min (same as that for integrator C-R3A (column C, silicon oil OV 1701 on fused silica (25 m) compound A; coinjection enhances this peak). Redistillation of fractions operated at oven temperature 70 O C , He). For pressure-dependent kinetic 2 and 3 gives compound C (retention time, 34 min): 4 g, 36%; 'H NMR measurements, a 7-kilobar vessel is heated by an external oil bath (C6D6) consistent with that of (E)-1,4-diphenylhexa- 1,5-diene, 7.26thermostated to f0.2 "C and pressurized by a hand-driven press (Nova7.03(m,lOH),6.33(d,lH,J= 15.8Hz),6.16-6.07(m,lH),6.01-5.92 Swiss). In order to follow the time dependence of a reaction at elevated (m, 1 H), 5.04-4.99 (m, 1 H), 3.32 (q, 1 H, J = 6.45 Hz), 2.59-2.48 (m, pressure, one exit of the vessel is connected to a valve having a fine 2 H). The residue from distillation, which crystallizes on cooling, is spindle, which allows the release of a small sample (ca. 100 pL) from the recrystallized from EtOH and affords 0.8 g (10.2%) of (E,E)-1,6pressurized reaction mixture (10 mL). The released samples are analyzed diphenylhexa- 1,Sdiene (compound G; retention time 49 min): mp 77.5by GC (column C). For measurements of densities a Densimeter DMA 78 O C (lit.i3amp78.8-79.2 "C); 'H NMR (C6D6) 7.36-7.16 (m, IO H), (Chempro) is used. 6 . 4 4 ( d , 2 H , J = 15.71Hz),6.30-6.24(m,2H),2.39(t,4H,J=3.32 trane4-Cyano-1,4-diphenylbutene-l.trans-Cinnamyl chloride (AldHz) . rich; NMR consistent with pure trans; 19.8 g, 0.130 mol) is slowly added To obtain information on the structure of the remaining compounds at 45 O C under argon to a stirred mixture of phenylacetonitrile (Aldrich; formed in the reductive coupling of trow-cinnamyl chloride, the following 17.45 g, 0.149 mol), 50% NaOH (46 mL), and benzyltriethylammonium experiments are performed. chloride (2 g). Stirring at this temperature is continued for 4 h and then Irradiationof(E,E)-l,6- and (E)-1,4-Diphenylhexa-l,S-diene (DPHD). at room temperature overnight. The reaction mixture is treated with 100 Asolutionof (E,E)-1,6-DPHD (50mg, 98.8%of purity, 1.2% 1,CDPHD) mL of benzene and 175 mL of water. The separated organic layer is and bibenzyl as sensitizer (3.5 mg) in benzene (3 mL) is irradiated with dried (MgS04) and concentrated in uucuo. Distillation of the residue a sun lamp (375 W) under argon while being cooled with water. Aliquots (bp 162-164 O C at 0.5 mmHg) gives 10.6 g (36%) of trans-4-cyanoare taken after 1.2, and 6 h and analyzed by GLC on column A. The 1,4-diphenylbutene-1, which crystallizes on standing at 0 OC. Recrysamount of (E)-1,4-DPHD stays constant within the run (1.I-1.2%). After tallization from MeOH gives pure nitrile: mp 45.5-46 "C;IR 2220; IH 1 h, the reaction mixture contains 97.5% (E,E)-1,6-DPHD and 1.4% of NMR (CDC13) 7.43-7.26 (m, 10 H), 6.52 (d, 1 H, J = 15.6 Hz), 6.19 compound E in a 97.5:1.4 ratio; after 2 h, the ratio is 95.8/3.1; and after (dt,lH,J=15.6H~,J=7.3H~),3.94(t,lH,J=6.7H~),2.87-2.77 6 h, themixtureconsistsof70.l%of (E,E)-1,6-DPHD, 26.2%ofcompound (m, 2 HI. E, and 2.5% of compound D. Based on the order of their appearance (E)-Z,S-Diphenylpeot-4-enal.A 1 M solution of diisobutylaluminum during the irradiation, compounds D and E are tentatively assigned the hydride (DIBAL) in toluene (Aldrich; 1OmL, 20% molar excess) is added structures (Z,Z)-1,6-DPHD and (E,Z)-1,6-DPHD, respectively. to a stirred solution of trans-4-cyano-1,4-diphenylbutene-l(3 g, 0.01 3 Irradiation of (E)-1,4-DPHD (30 mg, purity 99.5%, containing 0.2% mol) in benzene at 40 "C under argon over a period of 60 min. Stirring of compound B and 0.3% of (E,Z-)-1,6-DPHD) in 3 mL of benzene is continued at this temperature for an additional 2 h and then overnight containing 3 mg of bibenzyl gives, after 2 h of irradiation, a mixture at room temperature. The reaction mixture is quenched with MeOH consisting of 19.9% of compound B, 75.7% of (E)-1,4-DPHD, and 0.6% and water. The organic layer is washed twice with 5% sulfuric acid, of (E,Z)- and 0.3% of (E,E)-1,6-DPHD. Compound B accordingly is water, aqueous NaHC03, and water, dried (MgSOd), and concentrated tentatively assigned the structure (Z)- 1,4-DPHD. in uucuo. The crude reaction product (2.7 g) is purified by flash Heating (E)-1,4-Diphenyl(1W)hexa-l,S-dieneat High Temperature. chromatography on silicagel (elution with 15% ether in hexane) to give Aliquots of a 0.4% solution of (E)-1,4-DPHD in o-dichlorobenzene (20 1.1 g (33%) of pure aldehyde: IR 1725; IH NMR (CDC13) 9.78 (d, 1 mg in 5 mL) containing 11.1 mg of dihydroanthracene (15 molar equiv) H, J = 0.2 Hz), 7.10-7.5 (m, 10 H), 6.40-6.48 (d, 1 H, J = 15.8 Hz), are placed in several tubes. The tubes aredegassed, sealed under vacuum, 6.08-6.18 (dt, 1 H, J = 15.8 H z , J = 7.15 Hz), 3.67-3.75 (t, 1 H , J = and heated in a Techne FB-07 fluidized alumina bath at 250 "C for 15, 7.5H~),2.98-3.08(ddd,J=7.5 H z , J = 7.15Hz9J=7.8Hz),2.62-2.72 30, and 60 min and at 300 "C for 15 min. Two additional experiments (ddd, 1 H, J = 7.5 Hz, J = 7.15 Hz, J = 7.8 Hz); 13C NMR 199.88, at 250 O C for 60 min are performed in benzene in the presence of 1.5 and 137.11, 135.62, 132.24, 129.02, 128.80, 128.37, 127.61, 127.12, 126.50, 15 molar equiv of dihydroanthracene, respectively. GLC analysis on 125.98, 59.03, 33.23. column B gives the results shown in Table 7. (E)-l,+Diphenyl( l-1JC)hexa-1,5-diene.A solution of the aldehyde Reaction of (E)-1,4-Diphenyl(1W)hexa-l,S-dienein Thiophenol. A above (0.432 g, 1.53 mmol) in 5 mL of dry THF is added dropwise at solution of 300 mg of (E)-1,4-DPHD (purity 93.3%, the remaining 6.7% 0 "C to a stirred solution of triphenyl-'3C-phosphoniumylide prepared consisting of the other isomers of DPHD as described above) in 15 mL from triphenyl(13C)methylphosphonium iodide (0.9 g, 2.23 mmol) and of freshly distilled thiophenol (Aldrich) is placed in a Pyrex tube which n-butyllithium (Aldrich, 0.822 mL of a 2.5 M solution in hexane, 2.23 is then degassed, sealed under vacuum and heated in the vapors of boiling mmol) in dry THF at 0 OC. Stirring is continued at this temperature anisole (154 "C) for 15.2 h (1 half-life, as determined in the kinetic study for30min. Thereactionmixtureisdiluted with 15mLofTHF,quenched of therearrangement, uideinfru). The residueafter removal ofthiophenol with 15 mL of saturated aqueous ammonium chloride, and extracted by distillation is dissolved in ether (10 mL). The solution is washed with with methylene chloride. The organic layer is dried (MgS04), filtered, 5% NaOH and water, dried (MgS04), and concentrated in uucuo to a treated with ca. 2 g of silicagel, and concentrated in uucuo to a solid crude product (315 mg), which is flash chromatographed on silicagel. (product impregnated on silicagel), which is introduced at the top of a Elution with 2% ether-hexane gives a fraction (208 mg), GLC analysis of which on column A (oven temperature 210 "C, He 5 Ib) shows it to (37) Doering, W. v. E.; Bragole, R. A. Tetrahedron 1966, 22, 385-391. consist of ten compounds [retention times in min (yield in %)I: 18 (18); (38) Marvel], E. N.; Lin, C. J . Am. Chem. SOC.1978, ZOO, 877-883. 33 (33); 62 (33); 70 (6), and six minor compounds 2C-60 (2-3). The (39) Newcomb, M.; Vieta, R. S.J . Org. Chem. 1980, 45, 47934795.

W.V.E.D.belatedly takes an opportunity to accept the evidence for the irreproducibility of the alleged realization of "The Carbon Analogue of the Claisen Rearrangement of Phenyl Allyl Ether"37

Doering et al.

4296 J. Am. Chem. SOC.,Vol. 116, No. 10, 1994 Table 8. The IH NMR Spectra of l(6-1%) and 1(3-'%) Used for Analysis of Thermal Rearrangement l(6-W): 6-13C; 3-I2C l(3-1%): 6-'2C; 3-I3C

benzene-db0

toluene-dsO

4.75, 5.30; 2.50 5.05; 2.30, 2.75

4.73, 5.25; 2.48 5.00; 2.28, 2.70

In ppm from TMS. Table 9. Effect of Pressure on Relative Areas in IH NMR Spectra of 1(6-'%) and 1(3-'%) after Heating 1(6-l%) for 60 000 s at 162 o c

(E)-l,l'-Bi-3-phenylcyclohex-2-enylidene. A solution of 1.O g of 3-phenylcyclohex-2-enonea(mp 64-65 OC; lit.a mp 64.5-66 "C) in 10 mL of dry THF is added to a "titanium reagent", prepared from 5.5 g of TiCl4, 3.04 g of activated zinc dust, and 0.48 mL of dry pyridine in 50 mL of dry THF, and processed in the usual manner.35a The crude product is extracted with CHZC12, dried (NaZSOd), and concentrated in vacuo to a yellow solid (0.76 g) consisting of (E)- and (Z)-l,l'-bi-3phenylcyclohex-2-enylidenein the ratio 2/ 1, as determined by NMR. The solid is triturated in a mortar with 5 mL of THF to remove the much more soluble (Z)isomer. Filtration and washing with 2 mL of THF affords yellow crystals of (E)-1,l'-bi-3-phenylcyclohex-2-enylidene (BPC): 275 mg, 34%; >98% of purity; mp >203 OC dec; IH NMR (CsD.5;assignments based on decoupling experiments) 7.42 (d, 4 H, J =

7.3Hz(o,m),o-H),7.17(t,4H,J=7.3Hz(m,p),m-H),7.10(~,2H, lb 600ob

6.55 2.82

6.50 2.80

2.06 2.01

1.96 2.07

76523.5 57.9:42.1

5.31 15.33

le 6000'

6.39 2.79

6.29 2.63

1.99 2.06

1.99 2.03

76.1:23.9 57.0:43.0

5.41 16.40

In units of 1 P s-1.

b

In benzene-& c In toluene-d8.

GC-mass spectrum of this fraction (column DB1,15 m, oven temperature 100-250°C,He8 1b)showsthat,withtheexceptionofthefirstcompound ( m / z 153), all others have m / z 344, corresponding to addition products of thiophenol to DPHD: calcd for C24H24S. m / z 344. The complex NMR spectrum of this fraction shows a ratio of vinyl and methylene protons of 1:9, consistent with the majority of the addition compounds being derivatives of cyclopentane or cyclohexane. Kinetics of the Thermal Rearrangement of (E)-1,4-Diphenyl(6-l%)hexa-l$-diene (1(6-l%)) to (E)-l,4-Diphenyl(3-1%)hexa-1,5-diene (1(3-'%)). (e) At Atmospheric Pressure. In preliminary experiments, 2 aliquots of a 2.4% solution of the diene (97.4% of purity) in C6D6, each in a tube of Pyrex and lead-potash glass, respectively, are degassed, sealed under vacuum, and heated at 154 OC (anisole vapor) for 27 h. The ratio of educt to product, as determined by integration of the NMR spectra (uide infra), is essentially the same for both specimens: 79.7/ 20.3 (Pyrex) and 80.4/ 19.6 (lead-potash). Since the nature of the glass makes essentiallynodifference, all subsequent experiments are performed in Pyrex. Kinetic studies are effected in degassed deuteriobenzene (Cambridge Isotope Laboratory, 100% 3 freeze-thaw cycles) in vacuum-sealedNMR tubes (No. 528 Pyrex) with 18-crown-6 ether as internal standard. Heating is in the vapors of appropriate liquids boiling under reflux. The tubes are withdrawn periodically for analysis and returned for further heating. Quantitative analysis of the almost degenerate rearrangement is effected by taking advantage of I3Ccoupling with vinyl hydrogen atoms at C-6 and methylene hydrogen atoms at C-3 (dl%:5.21-5.15 (m) and 4.90-4.84 (m); and 2.59-2.48 (m); and 3-1%: 5.05499 (m); and 2.712.61 (m) and 2.45-2.35 (m)). Recovery is monitored by comparing the combined signals corresponding to the benzylic hydrogen atom at C-4 (3.32 (q), J = 6.45 Hz) and the singlet at 3.55 ppm in the standard. An acquisition time of 3.77 sand an interpulse delay time of 22 s is employed, 32-64 scans being accumulated for each measurement. Details of the calculation and corrections for compounds containing only I2C and two l3C are included as supplementary material in elaboration of the results in Table SI; results are shown in Table 2. (b) Under High Pressure. Two solutions each of 25 mg of (E)-1,4diphenyl(6-13C)hexa-1$diene in 2 mL of benzene-& and toluene-& respectively,were prepared. Each was divided and introducedwith sealing, one into polytetrafluoroethylene (PTFE) tubes that had been deactivated by prior treatment with triethylamine, the other into likewisedeactivated glass ampules. The four samples were then heated for 60 000 s at 162 OC: theglassampulesat 1 bar, theFTFEtubesat 6000bar. Quantitative analysis of 1(6-l%) and l(3-I%) was effected by IH NMR using the signals shown in Table 8. The resulting ratios given in Table 9 are translated into the single-point first-order rate constants by means of the followingequation: 2kt=ln([l]0-[1],)/([1] -[l],)where [ l ] o = 100, [l], = 50, and t = 60 000 s. From the pressure dependence of the single-point first-order rate constants listed in Table 5 , the activation volume was determined to be AV = -8.6 f 0.2 cm3 mol-l (derived from the linear part of the slope in the pressure range between 530 and 2020 bar) and -9.5 f 0.1 cm3 mo1-I (derived from the nonlinear slope over the whole range of pressure [uide infra]).

J 1.4 Hz, H-2), 7.07 (t, 2 H, J = 1.3 HZ @,o), p-H), 2.44 (t, 4 H, J = 6.3 Hz, H-6), 2.36 (t, 4 H, J = 5.8 Hz, H-4), 1.62 (quintet, 4 H, J = 5.8 Hz, H-5); 13CNMR (CDCl3) 160.6, 160.0, 154.9, 154.2, 128.5, 128.0, 125.6. (Z)-l,l'-Bi-3-phenylcyclohex-2-enylidene: 1H NMR (C&, assignments based on decoupling experiments) 7.50 (s, 2 H, J = 1.4 Hz, H-2), 7.38 (d, 4 H, J = 7.3 Hz (o,m),o-H), 7.10 (t, 4 H, J = 7.3 Hz (m,p),m-H), 7.02 (t, 2 H, J = 1.4 Hz @@),pH), 2.36 (t, 4 H, J = 6.0 Hz, H-4), 2.28 (t, 4 H, J = 6.3 Hz, H-6), 1.68 (quintet, 4 H, J = 6.3 Hz, H-5); I3CNMR (CDCl3) 161,155, 154, 151, 128,124, 124. Kinetics of the Interconversion of (E)- and (Z)-l,l'-Bi-3-pbenylcyclohex-2-enylidenes 7 and 8. For the kinetic experiments, the NMR resonances of the methylene group in 7 and 8 at C-6 alone are suitable (2.28 and 2.44 ppm, respectively). The intervening resonances of the methylene groups at C-4 are both at 2.36 ppm and overlap very slightly but equally. A solution of -3 mg of 7 in 1 mL of C6D6 (Cambridge Isotope Laboratories; " 100%" grade) is sealed under vacuum in a Pyrex NMR tube and heated in vapors of boiling liquids of appropriate boiling point. Rateconstants arecalculated by a nonlinear least-squaresprogram that allows the starting concentration, XO,the equilibrium concentration, xe, and (kl + k-1) to vary independently as a function of concentration, x, at time 1. The primary data are given in Table SI1 as supplementary material. Specificrateconstants and equilibriumconstants and thederived Arrhenius and Eyring parameters are given in Table 6. Pressure-Dependent Kinetics of the Thermal Rearrangement of cis 1,2-Divinylcyclobutane (cis-2) to 1,iCyclooctadiene (3) and the Rearrangement and Fragmentation of hrursF1,2-Divinylcyclobutane (trans-2) to 3,4-Vinylcyclohexene (4) and 1,SButadiene (5). A mixture of cis-2, trans-2, and 4 was prepared photochemically from butadiene following the procedure of Hammond et a1.'1 in the 15:78:7 ratio, respectively (lit.41 17:76:7). After separation by GC on a column packed with 20% silicone oil DC 550 on Chromosorb P (70 OC, He), tran9-2 and 4 were isolated each in >99% purity, while cis-2 and 3, the product of some rearrangementon thecolumn, wereobtainedina 90.29.8 ratio. Response factors were determined by GC analysis on column C of a solution consistingof h m - 2 (28.9 mg, 0.267 mmol; retention time (r.t.) 3.8 min), 3 (57.3 mg, 0.530 mmol; r.t. 10.1 min), 4 (45.6 mg, 0.422 mmol; rut.5.6 min), and n-nonane (38.7 mg, 0.302 mmol; r.t. 6.8 min) in n-heptane (370.6 mg, 3.70 mmol; r.t. 3.1 min). The following response factors were obtained: ~ 2 , 0 . 9 5 9 . f 0 . 0 0 3 , 3 , 0 . 9 7 2 f 0 . 0 1 5 , a n d 4 , 0 . 9 8 8 f 0 . 0 0 5 ( c b 2 was assumed to be the same as -2). Portions of a solution containing the mixture of c i s 2 and 3 obtained above (204.5 mg, 1.89 mmol) and n-nonane (198.3 mg, 1.55 mmol) in n-heptane (100 mL) were placed in the 7-kbar vessel and heated at 69.8 OC and the various pressures indicated in Table 3. At each pressure, the extent of rearrangement of c i s 2 to 3 was followed by GC to 70-80% of conversion. Recovery of c i s 2 and 3 as measured against the internal standard was >99%. Specific rate constants shown in Table 3 were calculated from the time dependence of the disappearance of c i s 2 according to the first-order rate law of irreversible reactions. Portions of a solution containing t m w 2 (102.9 mg, 0.95 1 mmol) and n-nonane (128.5 mg, 1.00 mmol) in 100 mL of n-heptane were placed ina 7-kbar vessel and heated at 159.6 OC and the various pressures shown in Table 4. The reaction of trane2 to 3,4, and 5 was followed by GC to 50-6036 of conversion (at least six different times each). Since the analysis of butadiene 5 was not reproducible owing to its low boiling point, the amount of 5 was estimated as the difference between 100%and the recovery of bpne2 to 5 4 , and 5 by GC against the internal standard. At each pressure the sum of the specific rate constants ( x k l ) was

-

(40) Woods,G. F.;Tucker, I. W. J.Am. Chem. SOC.1948,70,2174-2177, 3340-3342. (41) Hammond, G. S.;Turro, N. J.; Liu, R. S.H. J. Org. Chem. 1963,28, 3297-3303.

Rearrangement of 1,4-Diphenylhexa- lJ-diene

J. Am. Chem. SOC.,Vol. 116, No. 10, 1994 4297

Table 10. Temperature Dependence of Partial Molar Volumes (V, cm3 mol-') in n-Heptane T, OC

V(fnns-2)

20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0' 60.0' 65.0' 70.0'

140.0 140.9 142.2 142.7 143.4 143.9 144.7 145.6 146.5 147.1 148.3

V(ci5-2)' 35.4 36.4 36.8 37.6 38.4 39.1 39.9

V(3)bc

V(4)d

122.6 123.1 123.7 124.1 124.5 124.9 125.3 125.8 126.3 126.7 127.3

130.4 131.1 131.8 132.5 133.2 133.6 134.3 135.0 135.8 136.4 137.1

- --

At temperatures higher than 50.0 O C , the rearrangement of cis-2 3 takes place with measureable rate. K Z O ~= 3.894 & 0.007 (cis-2 3). ~ 2 =0 3.829 ~ & 0.028 ( t r a m 2 3). K" = 2.526 & 0.039 (trans-2 4). e K20 in units of 10-3 K-I.

-

determined from the time dependence of the disappearance of t"2. The individual specific rate constants in Table 4 were obtained by a Marquardt optimization The volume of activation ( A V ) was calculated from the linear part of the slope of the correlation of In kl against p (In kl = a1 + blp; AV = b1RT)j3 The linearity holds up t o p = 1 kbar for the reaction of cis-2 to 3 (Table 3) and 3 kbar for the reaction of trans2 to 3,4,and 5 (Table 4) (for results, see Scheme 2). For the reaction of cis-2 to 3, AV' was also derived by a quadratic equation (In kl = a2 + b$ + czp2; AV = -b2RT), which is valid over the entire range of pressures and gives rise to the value AVI = -13.4 f 0.6 cm3 mol-', in agreement within experimental uncertainty with the value from the simpler treatment. (42) Marquardt, D. W. J. SOC.Ind. Appl. Math. 1963,11,431-441. We thank Dr.R. Fink for a copy of the program KINETIC, which permits optimization by the Marquardt procedure of kinetic schemes with up to seven components. The very significant advantage of this procedureis its estimation independently of the uncertainties in each individual rate constant of the kinetic scheme. (43) Asano, T.; Okado, T. J. Phys. Chem. 1984,88, 238-243.

Reaction Volumes. Partial molar volumes (V) of trans-2, 3, and 4 were determined from the density (d)of solutions measured for various concentrations (c) and temperatures listed in Table 10 by using the followingequation: @ = (M/d0)[(1000/c) X (d-do)/do] (M, molecular weight; do, density of pure solvent)," a linear extrapolation of @ to c = 0. V(cis2) was determined from the density of a solution containing a mixtureof 90.2%cis2and 9.883. ThereactionvolumesshowninScheme 2 are calculated from the partial molar volumes in Table 10: AVR = V(product) - V(educt). From the temperaturedependenceofthereaction volumes, the temperature coefficients ~ 2 can 0 be calculated with the equationgiven by El'yanovet al.,'s A P ( T ) = AVR(To)[l + KZO(TTO); TO= 20 O C . The ~ 2 values 0 listed in Table 10 are of the same order of magnitudeas thegeneralvalue ( ~ 2 0= 4.4 X K-') reported by El'yanov et al.45

Acknowledgment. The investigations of W.V.E.D.,L.B., K.S., and J.H.T. have been supported by NSF Grants CHE-85 1845 1, CHE-88 16186,andCHE-9123207. F.-G.K. and J.-S.G. express their thanks to the Deutsche Forschungs Gemeinschaft, the Ministry for Science and Research of the Land NordrheinWestfalen, and the Fond der Chemischen Industrie for the support of their part of the investigations. Supplementary Material Available: Table SI and Table SI1 contain the unprocessed experimental data from which Tables I1 and VI are derived, respectively. Details of the calculations of the kinetics of the rearrangement of l(6-W)(Scheme 1) are also given (14 pages). This material is contained in many libraries on microfiche, immediately follows this article in the microfilm version of the journal, and can be ordered from the ACS; see any current masthead page for ordering information. (44) Le Noble, W. J.; Asano, T. J. Am. Chem. SOC.1975,97, 1178-1 181, (45) El'yanov, B. S.; Gonikberg, E. M.J. Chem. Soc., Faraday Trans. 1 1979, 75, 172-191. El'yanov, B. S.; Vasylvitskaya, E. M. Reu. Phys. Chem. Jpn. 1980,50, 169-184.